Because of their outstanding w ear-resistant characteristics ceramic materials have found increasing application in internal combustion engines and similar environments where continuous contact between engine components occurs during engine operation. Valve actuating mechanisms, in particular, have surfaces which are subject to frequent contact with other engine components. Depending upon the type of engine in which the valve actuating mechanism is located, these contact surfaces may be located on a cross-head assembly, on a tappet or on a rocker arm. Because such surfaces are subjected to a high degree of contact, they should be formed of a material characterized by high abrasion and wear resistance which will not fail during the life of the engine component. Materials having these characteristics are often not suitable for use in forming the entire engine component and, therefore, must be secured in some manner to the engine component to form the contact surface.
Materials which provide a durable, abrasionresistant surface for this purpose are known in the prior art. For example, U.S. Pat. No. 4,485,770 to Saka et al. describes such a material. The material disclosed in this reference is an iron-based sintered alloy which is pressed to the desired shape, sintered and then heat treated to produce a pad having a predetermined alloy structure. The pad thus formed must be attached in some manner to the engine component contact surface. The Saka patent, however, is silent on exactly how this is accomplished in a manner which produces a durable, effective abrasion-resistant surface.
An effective wear resisting insert for the camcontacting surface of an engine tappet formed from a highly wear resistant ceramic material such as silicon carbide is disclosed in U.S. Pat. No. 4,366,785 to Goloff et al. The ceramic insert disclosed in this patent is press-fitted into a recess in the tappet rather than permanently bonded or secured to the tappet and, consequently, unless the proper interference is achieved, the ceramic insert will not remain attached to the tappet.
Although ceramic materials are generally preferred over metal alloys for the formation of engine component contact surfaces because of their superior abrasion and wear-resistant characteristics, the attachment of ceramic contact surfaces is more difficult because the engine components to which they are attached are usually formed from metal. The different coefficients of thermal expansion of metals and ceramics causes the metal-to-ceramic joint to be stressed and ultimately results in the separation of the contact surface from the engine component. Methods for creating an effective metal-to-ceramic joint have been disclosed by the prior art. U S. Pat. No. 3,034,205 to Ames is exemplary of these prior art methods. Ames discloses an alloy consisting of copper, silver, indium, nickel and titanium which can be employed to braze a metal, particularly stainless steel, to a ceramic by heating to a temperature of about 1800.degree. F. to 200.degree. F. for at least five minutes, preferably in an atmosphere of cracked ammonia gas. U.S. Pat. Nos. 2,857,663 to Beggs; 3,091,028 to Westbrook et al. and 3,226,822 to Budde et al. also disclose methods of bonding a ceramic and a metal. However, none of the aforementioned patents discloses the formation of a ceramic-to-metal bond which would be likely to withstand the stresses encountered by a contact surface in an engine environment.
The metal engine components which support the above-described contact surfaces are subject to stresses during engine operation and therefore, must themselves be strong, durable and capable of meeting specified mechanical and physical characteristics. Depending upon the material from which these components are formed, such characteristics may be achieved by heat treatment. In the case of a cast iron component, heat treatment followed by cooling will yield the required microstructure and properties. It would be highly desirable and economical to heat treat the metal engine component and attach a wear-resistant ceramic surface element in a minimum number of process steps. U.S. Pat. No. 3,615,920, to Talento discloses a combination heat treatment and braze cycle for metal which results in the modification of its microstructure and produces desirable mechanical properties in the metal thus treated. There is, however, no suggestion in this reference that the multi-step method method described therein could be applied to metals other than stainless steel or that the braze cycle portion of this method could be employed to attach a ceramic material to the metal.
Consequently, the prior art fails to show a wear-resistant ceramic contact surface element permanently secured by brazing to a metal internal combustion engine component wherein a single process simultaneously brazes the ceramic element to the metal and achieves the desired microstructure and mechanical properties in the metal forming the engine component.